The present invention relates generally to apparatus for doming the bottom walls of cans and, more particularly, to apparatus for doming thin walled aluminum can bodies of the type having a cylindrical side wall and an integrally formed bottom wall.
Metal containers such as cans which are adapted to hold contents under pressure are often provided with a upwardly extending dome in the bottom wall thereof to resist the tendency of the bottom wall to deform excessively under pressure and also to provide a generally planar annular portion at the periphery of the bottom wall which provides a stable support base for the can. Numerous domed containers are described in prior art patents such as U.S. Pat. Nos. 1,963,795; 3,904,069; and 4,037,752 which are hereby incorporated by reference.
In doming the bottom of relatively thin walled metal cans, such as conventional aluminum beer cans, a continuing problem has been the formation of radially extending crease lines in the domed portion of the can. These crease lines are probably formed as a result of non-uniform deformation of the can bottom wall at the time it is initially contacted by a dome-shaped die assembly. The non-uniform deformation may be due to the fact that the die assembly initially makes a point contact at the center of the can bottom resulting in an initial deformation of the can bottom into a conical configuration. It is in the transition of the can bottom from a generally planar shape to such a conical shape that radial creasing of the can bottom takes place. Such a creased dome configuration is generally known in the art as a "flower dome." A problem with flower dome formation, other than the generally aesthetically unacceptable appearance, is that the crease lines may rupture or weaken the can bottom and may cause leaks or non-uniform deformation of the can bottom when the can is pressurized. Another problem associated with dome formation in integrally formed thin walled can bodies is that the deformation of the can bottom wall during doming tends to cause metal flow from the can lateral side wall to the can bottom wall resulting in a slight axial shortening of the can. One prior art technique for eliminating these problems has been to tightly engage a peripheral portion of the can bottom wall and a lower portion of the can side wall between a bodymaker punch assembly and a pressure ring during dome formation. Such a peripheral engagement of the can wall tends to stabilize the bottom wall circumferentially, thereby reducing the tendency of the bottom wall to crease during dome formation. Such a peripheral engagement also tends to limit the flow of metal from the can side wall to the can bottom wall. Another prior art method, sometimes used in combination with a pressure ring, for eliminating flower dome formation is application of relatively high pressure to the domed region of the bottom wall during dome formation to "iron out" any creases that may have been formed during the initial portion of the doming operation. A problem with the former technique is that, in applying sufficient pressure to the periphery of the can bottom to prevent the undesirable effects of can shortening and flower dome formation, the engaged portion of the can bottom is sometimes damaged by the pressure ring. A problem with "ironing out" radial creases is that the ironed out creased area has different strength and deformation characteristics than the other portions of dome. Furthermore, such ironing out techniques are not always successful in removing all of the radial creases.